1. Technical Field
The present invention relates to sensors for determining the concentration of gases. More particularly, the present invention provides methods which permit the selection of a specific metal oxide which, in turn, is selective in its sensitivity to a specific gas. A sensor, which includes the metal oxide in the form of a thin film in conjunction with a substrate and electrodes, may then be produced which is capable of rapidly detecting very low concentrations of the specific gas with reduced interference from other gases. The sensors thus produced may be utilized in measuring gases in a variety of settings, including automotive and biological applications.
2. Background of Related Art
Sensors are utilized in a variety of applications to determine the presence of gases. For example, ammonia sensors are being used in diverse applications such as food technology, chemical plants, medical diagnosis, and environmental protection. There are several challenges associated with the development of sensor technology for monitoring gases, such as those found in combustion processes or in biological systems. With respect to combustion processes, the challenges include high temperatures, presence of reducing and oxidizing gases, organic vapors (VOCs), high flow rates, etc. With biological systems, challenges include sensitivity to extremely low levels of gases, presence of reducing and oxidizing gases, organic vapors (VOCs), etc. One of the more difficult challenges in these processes is achieving selectivity for a specific compound or gas.
Metal oxides have been utilized in sensors for some time. These oxides may be found as several different crystallographic phases with identical chemical compositions but different crystal structures (polymorphs). Metal oxides used in sensors include, for example, titanium dioxide (TiO2), tungsten trioxide (WO3), molybdenum trioxide (MoO3), vanadium pentoxide (V2O5), zirconium dioxide (ZrO2), niobium pentoxide (Nb2O5), iridium dioxide (IrO2), tantalum oxide (Ta2O3), and combinations thereof.
Sensors frequently use semiconductors for the qualification and/or quantification of a compound or substance being detected. There are two basic types of semiconductors: n-type, in which the density of holes in the valence band is exceeded by the density of electrons in the conduction band; and p-type, in which the density of electrons in the conduction band is exceeded by the density of holes in the valence band.
There have been many reports on p-n type transitions occurring during the testing of resistive type sensors. The conductivity of a semiconductor can be represented in terms of the carrier density and mobility of the individual charge carriers by equation (1)σ=−qnμn+qpμp  (1)where n and p are the number of electron and hole carriers respectively, q is the charge associated with the charge carrier and the μ values represent the corresponding mobilities. All four parameters are dependent on temperature and the values of n and p, which determine whether it is an n-type or p-type semiconductor, vary with the generation of inter-band traps due to the formation of vacancies or impurity substitution.
For example, MoO3 is an n-type semiconductor in the stable state. In a metastable state, however, oxygen vacancies may exist in different proportions leading to either excess electrons near the conduction band or holes near the valence band. A typical oxygen vacancy formation may be represented by the following quasi-chemical reaction (2):Oo*⇄½O2(g)+Vo2++2e−  (2)
Where Oo* represents an unstable oxygen atom in an oxygen site and
Vo2+ represents an oxygen vacancy with double positive charge.
When oxygen is incorporated into these vacancies, a reversible reaction (3) occurs as shown below.[Vo2+]+½O2⇄Oo+2h+  (3)When the hole concentration drops below a threshold value or when equation (2) is favored to equation (3), p to n shift occurs due to formation of donors near the conduction band.
The simple nature of the sensing mechanism of semiconducting oxide gas sensors often results in a given oxide system being sensitive to more than one type of gases, which causes undesirable gas interference effects to the sensing behavior of the sensor. For example, previous efforts investigating the sensing response of MoO3 to various gases in the temperature range of 250° C. to 475° C. have revealed that MoO3 was more sensitive to NH3 than to NO2 and H2 at 425° C. and that the gas sensitivity dropped with decreasing film thickness (<300 nm). Multilayer sputter processing of MoO3 resulted in improved H2 sensing properties and low cross-sensitivity towards NH3. See Imawan et al., Sensor Actuat B-Chem, 78, pp. 119–125 (2001). Others have reported the enhancement of sensitivity and selectivity to NH3 following the addition of Ti overlayers to MoO3, and to H2, by adding V2O5 to MoO3 (Imawan et al., Sensor Actuat B-Chem, 64, pp. 193–197 (2000)), while others have reported sensitivity towards CO for Ti additions to MoO3. Ferroni et al., Sensor Actuat B-Chem, 58, pp. 289–294 (1999).
Similarly, there have been numerous reports of WO3 sensors for NOx detection. Some reports describe thick film WO3 sensors that are sensitive to NO2 at 100° C., but these films showed a very weak response to NO2 above 250° C. and the response was found to be p-type at higher temperatures (>250° C.). Chung et al., Sensor Actuat B-Chem, 60, pp. 49–56 (1999). Other workers have fabricated sensor arrays for the selective detection of NO2 and NH3; these sensors were operable at optimum temperatures of 300° C. and 350° C. and utilized dopants to achieve selectivity (Marquis et al., Sensor Actuat B-Chem, 77, pp. 100–110 (2001)). Still others report WO3 thin films sputter deposited at 350° C. have shown good response to NOx at 400° C. Sberveglieri et al., Sensor Actuat B-Chem, 26, pp. 89–92 (1995).
The addition of dopants and other treatments, such as heating, are widely used approaches to stabilize metal oxides used in sensors. For example, U.S. Pat. No. 6,173,602 describes a transition metal oxide gas sensor which includes a substoichiometric molybdenum trioxide of formula MoO3-x wherein MoO3 has been reduced by a thermal treatment or by substituting some of the molybdenum with a metal with a principal valence of less than six in order to stabilize the structure of the substoichiometric phase (MoO3-x).
Efforts are underway to develop sensors that are selective in their response to specific particular gases. These sensors could have use in numerous fields, including automotive and similar combustion applications, biological monitoring systems, environmental monitoring systems, etc.
In the automotive field, ammonia(urea)/Selective Catalytic Reduction (SCR) is one of the leading NOx emission reduction systems under consideration for diesel and lean-burn engines. SCR systems are employed in the exhaust systems of vehicles, composition systems in power plants, and in industrial boilers to monitor emissions of NO2 and NO. These gases are harmful by-products of combustion processes.
In a SCR converter, ammonia serves as a reducing agent for nitrogen oxides, such as nitrogen dioxide, converting them into environmentally safe nitrogen and water vapor. Adjusting the requisite stoichiometric ratio of nitrogen oxides to ammonia, or to some substance such as urea that can be converted into ammonia, can be done with satisfaction only if the nitrogen oxide concentration in the exhaust or flue gas can be measured. A selective ammonia sensor located downstream of the SCR catalyst may be utilized to calculate the amount of un-reacted and excess ammonia, which is fed into the inlet stream, thus minimizing possible ammonia and NOx emissions. Similarly, a sensor able to detect NO2 in the presence of NH3 would be extremely useful.
SCR systems have the potential to reduce NOx emissions by more than 90% with little impact on fuel economy. As the 2007 Tier II emission standards promulgated by the United States Environmental Protection Agency require over 90% NOx conversion, the automotive industry is actively developing control systems utilizing urea/SCR to meet these future standards. Transient control of the ammonia injection system is an essential part of the overall control system utilizing urea/SCR.
U.S. Pat. No. 5,546,004 describes a sensor for SCR systems used to measure the concentration and adjust the ratio of ammonia (urea) to nitrogen oxides. The sensing device is a non-selective sensing device in which oxide dopants are added to improve its sensitivity to ammonia, with titanium dioxide functioning as the main sensor material. Pairs of electrical contacts are disposed throughout the sensor material, with a course of concentration of an adsorbent being determined as a function of its penetration into the sensor material.
Biosensors are electronic devices used to detect the presence and determine the concentration of substances of biological interest. The use of enzymes in bio-detection adds selectivity to the sensing process (e.g. glucose oxidase membranes are used to monitor glucose levels of diabetics). See, e.g., Livage, et al., “Encapsulation of biomolecules in silica gels”, J. Phys.: Condens. Matter, 13, pp. R673–R691, 2001.
For example, U.S. Pat. No. 5,858,186 discloses a urea biosensor for hemodialysis monitoring where the sensor is based upon measurement of the pH change produced in an aqueous environment by the products of the enzyme-catalyzed hydrolysis of urea.
The sensitivity and selectivity of a biosensor depends upon the biologically active material, or receptor, included therewith. Suitable receptors for use in biosensors include enzymes, antibodies, lipid layers, cells etc. One drawback with current biosensors is the transducer, or detector, is often not selective, and thus false readings are common.
Current advances in the field of chemical sensing focus on liquid phase chemical detectors/biochemical devices, as well as optical and opto-electronic sensors, polymer-based or silicon-based. These competing sensor technologies primarily operate at temperatures ranging from room temperature to 250° C. and in relatively clean environments. However, sensors are still needed that are capable of operating at high temperatures (>400° C.) and in harsh conditions, e.g., those which prevail in catalytic processes involving nitrogen dioxide and ammonia synthesis or reduction.
Commercially available ammonia sensors suitable for use in the automotive exhaust environment are not yet available. Similarly, economical biosensors which are sensitive, selective, and stable are not readily available. Therefore, the development of selective sensors would be very important for development of these systems. Such a sensor should be inexpensive, and simple to fabricate and use.